A slow but steady snowfall turned into freezing rain as temperatures rose last night, making roads and bridges hazardous for motorists in the Washington area.

Earlier yesterday, snow produced predictable problems for commuters and a day off for about half of the area's school children. But by nightfall, the snow and the problems were less severe than predicted.

The snow, which started about 2:30 a.m yesterday, fell so slowly that less than a quarter of an inch had accumulated by the morning rush hour and only half an inch by evening, according to weather observers at National Airport.

Forecasters said the freezing rain was the result of an advancing warm front from the Gulf of Mexico that blanketed a four-day cold front leaving the area and heading into the Atlantic Ocean. Warm air atop cold air caused rain that froze when it reached the ground, they said.

Enough snow fell early yesterday; police said, to slow traffic throughout the area, and to cause major tie-ups on two of the area's major commuter roads -- Rte. 270 in upper Montgomery county and Shirley Highway in Virginia.

The snow caused a substantial increase in traffic accidents, but none of them was serious. Many buses ran 15 to 20 minutes behind schedule in both morning and evening trips, Metro officials said.

Government offices remained open, but absenteeism was high. Officials noted though, that yesterday was the end of the 1978 "leave year" for federal and D.C. workers, meaning those who didn't take leave by yesterday would lose it if they had accumulated more than 240 hours.

"People had two good reasons to stay home today." remarked one supervisor in the Agriculture Department. "A lot of them did."

Students had their first snow-day off of the winter in Montgomery, Prince George's and Loudoun counties. But the other school systems in the area remained open.

"The forecast was for 3 inches of snow, and conditions in the northern part of the county seemed pretty bad," said John Aubuchon, public information director for Prince George's county schools, whose classes were canceled. "We had to make a decision for the whole county, which means the worst part of the county essentially. So we decided we had to close."

Aubuchon said the decision was reached by Superintendent Edward J. Feeney at about 5:30 a.m.

In Fairfax County, where schools stayed open, school officials said they relied on advice from the state highway department that the snow was light enough so roads could be kept open to traffic.

"We had no big troubles," said William Shadle, the Fairfax school system's director of support services. "For some of the elementary schools, the buses arrived about a half hour late, but all in all things went well."

Shadle said cars slammed into four of the county's 700 buses, but none of the accidents caused any injuries or major delays.

Yesterday's snowfall was the first of the season to affect rush-hour commuters. On Nov. 26 and 27 about three inches of snow fell on a Saturday night and Sunday morning, the weather service recalled, but it melted quickly.

Last evening forecasters said they expected the snow to accumulate a bit more before turning to sleet and freezing rain. They said they expected rain on Saturday, with temperatures rising to about 40 degrees.